1. Field of the Invention
The present invention generally relates to a gear of a resin, such as an injection molded resin gear, for use in power transmission devices for various precision instruments, such as color copying machines, color printers and video tape recorders, various automotive parts and industrial equipment. The invention also relates to an injection molded resin rotating body, such as an injection molded resin gear, roller, pulley or sprocket, which may be often used in power transmission portions. Moreover, the invention relates to an injection molded article which is used as a dynamic injection molded resin rotating body, such as an injection molded resin gear or roller, or as a stationary shaft supporting member.
2. Description of the Prior Art
For example, tandem type color copying machines and color printers are designed to sequentially carry out steps of forming images of black (K), magenta (M), cyanogen (C) and yellow (Y) in a direction, in which a transfer material, such as a copy paper, is carried, to sequentially superpose and transfer images of different colors to the transfer material, which is carried by a carrier belt, to print a desired color image on the transfer material. In such color copying machines and color printers, an image forming unit for each color (K, M, C, Y) and a carrier belt are connected to a motor via gears to be operated by the driving force of the motor. Therefore, in order to enable a high quality color print, it is required to precisely operate the image forming unit for each color (K, M, C, Y) and the carrier belt to precisely transfer each color image to the transfer material. For that reason, the precision of gears for transmitting the rotation of the motor to the image forming unit and carrier belt is important. That is, if the precision of the gears is bad, the rotation of the motor is not precisely transmitted to the image forming unit and carrier belt, so that operation failure, such as irregular rotation, is caused in the image forming unit and carrier belt, thereby causing print failure, such as color deviation.
Users have requested to reduce the size, weight and price of the above described color copying machines and color printers and to reduce operation noises thereof.
Therefore, in the above described color copying machines and color printers, an injection molded resin gear, which has a higher precision than that of a cut metal gear and which can more greatly reduce the weight and operation noises than those of the metal gear, has been widely used as a gear for transmitting the rotation of a motor to an image forming unit or a carrier belt.
In a process for forming such an injection molded resin gear, a molten resin injected into a cavity shrinks when it is cooled to be solidified. The amount of shrinkage of the molten resin in a thick region of the resin is larger than that in a thin region thereof. As a result, for example, in an injection molded resin gear 31 as shown in FIG. 10, there are inherent problems (problems which are not caused by cut metal gears) in that the diameter of the connecting portion of a web 32 to a rim 33 is more greatly reduced than that of a portion of the rim 33 on the end side thereof, so that a shrink mark 34 is produced in a substantially central portion in face width directions to deteriorate the precision of the gear (e.g. total alignment error). Therefore, in order to further improve the precision of the gear, it is important to reduce the difference in shrinkage between parts.
As shown in FIG. 11, the inventors have proposed an injection molded resin gear 31 wherein a web 32 has a plurality of circumferential ribs 36, 37 and 38 arranged concentrically with a hub 35, at least one 37 of the plurality of circumferential ribs 36–38 being arranged so as to face a plurality of injection molding pin point gates 40 (see Japanese Patent Laid-Open No. 8-25501). This conventional injection molded resin gear 31 is formed by allowing most of a molten resin, which is injected from the pin point gates 40, to flow along the circumferential rib 37 in circumferential directions and by allowing the molten resin to flow inwardly and outwardly in radial directions from the circumferential rib 37. Thus, the flow of the resin is equalized in radial directions as a whole to make distribution in rate of shrinkage uniform, so that the gear is formed so as to have an excellent roundness. However, in recent years, with the innovative improvement of performance of precision instruments and so forth using the injection molded resin gear 31, it is desired to provide more precise injection molded resin gears.
Similar to the above described injection molded resin gear 31, it is desired to develop a technique for more precisely forming the shape of an injection molded resin rotating body, such as an injection molded resin pulley, sprocket or roller, which forms a part of a power transmission mechanism, to more precisely and smoothly transmit the rotation thereof.
In precision instruments and so forth using the injection molded resin gear 31, it is required to precisely support a dynamic (sliding or rotating) shaft or a stationary (fixed) shaft on a frame or the like. In such a case, there is used a shaft supporting member (an injection molded article) which is injection-molded so as to substantially have the same shape as that of an injection molded resin roller. The outer cylindrical portion of this shaft supporting member is designed to be fixed to a frame or the like, and the inner cylindrical portion thereof is designed to support thereon a shaft. The shaft supporting member does not rotate unlike injection molded resin rotating bodies, such as injection molded resin gears and rollers. However, it is desired to improve the precision of the shape of the shaft supporting member in order to prevent the deviation in alignment of the shaft and so forth.